Can you describe what exactly has broken
If Im right these Sony amps used a switch which has
a metal section coming out and had a outer round
part that just covered that metal.

It might be possible to super glue the little round section
back on if thats all that has broken.

If the whole thing is broken ? you will need to firstly
disconnect power cord from wall socket and carefully
remove the switch mechanism , sometimes the easiest
way to do this is to partly remove the front panel so
you can get better access to it - they might have either
soldered the board to a circuit board or it may have wires
that may need resoldering once you get a new or
replacement switch. In any case make a drawing
diagram showing all the connections. If Sony are
switching the mains you will find that active is being
switched so take care to wire it back the same way
and that you adequately insulate the mains from
touching anything else. If needed get help from
an electrician to check your work.

There are a few alternatives:
Can you tell me if its the same as this image of a TA1066

If so it looks fairly similar to what Pioneer also used
in their amps.

Get back to me and I will try and source a similar one for you

Sony spare parts might be able to help, but
they usually need a part number before they
get interested. In which case you would need
to visit a repair shop to hopefully find a manual
for that model - ie unlikely